1. Field of the Invention
This invention relates to photographic cameras, and, more particularly, to an electromagnetic control device constructed by employment of a core material having rectangular hysteresis curve magnetizing characteristics.
2. Description of the Prior Art
It is known to utilize electromagnets in constructing actuating devices for actuating various mechanisms of a camera from the shutter cocked positions and for controlling the period of actuation thereof in cooperation with electronic control devices therefor. For easier management of a battery operated camera with the decreased bulk and weight thereof, it is generally required to select a battery of small capacity, i.e. providing a low voltage and weak current, for employment as the electrical power source of the camera. With this regard, the electromagnet must be designed for correspondingly limited output power and energy consumption. In order to secure reliable performance of the electromagnet under the circumstances of this limitation, a solution has been proposed and found practical use in initiating a release of one or more of the camera mechanisms from the cocked position. According to this proposal, before a control pulse is applied to the solenoid of the electromagnet to initiate the release of the mechanism, the armature of the electromagnet which is operatively connected to the control member for the mechanism is previously held in the cocked position.
There are two methods of maintaining the armature in the cocked position, one of which is to energize the solenoid until a deactuating pulse is applied thereto, and the other of which is to utilize a resetting member for the mechanism in assisting the electromagnet to operate in such a manner that as soon as the resetting member is caused to escape from the path of movement of the control member of the mechanism in timed relation to the initiation of the energization of the solenoid, while still leaving the armature in the cocked position. With this arrangement, it is possible to minimize the period of energization of the solenoid and to reduce the energy consumption of the battery. In application to magnetic shutter control systems, the first-named method, however, has a disadvantage of consuming electrical energy at a high rate particularly when long shutter intervals are selected, as for example, in making bulb photography because the solenoid of an electromagnet controlling closing operation of the rear curtain of the shutter must be energized at least during the entire length of exposure time. In the case of the second-named method, although the disadvantage of premature consumption of the battery can be eliminated, it is unavoidable to increase the complexity of the magnetic control device and to employ means for establishing the timed relation mentioned above in operative connection therewith, or otherwise the satisfactorily accurate and reliable actuation control can not be effected. This in turn, calls for an increase in the product cost and the possibility of occurrence of damages of the combined device and means.